Optical fibers, such as fused silica optical fibers comprised of center cores of GeO.sub.2 -doped silica and cladding layers of silica, will waveguide light in the cores due to the lower index of refraction of the cladding layers. In the communications industry, optical fibers are being utilized to transmit information in the form of modulated coherent light beams. To reduce distortion of the transmitted light beam, optical fibers having cores of approximately 1 to 5 micrometers (microns) in diameter have been developed. To provide for ease of handling, the cladding layers are approximately 100 micrometers in diameter.
The optical fibers are further covered with a protective polymer coating about 10 to 15 micrometers thick to prevent surface cracking of the fibers. The optical fibers as commercially available have protective layers of polymers such as polyvinylidene chloride which are easily applied to the fibers. These polymers generally have indexes of refraction lower than the cladding layer which permits waveguiding in the cladding layers as well as the cores.
Waveguiding in the cladding layers presents a problem when the small diameter cores of the optical fibers are to be coupled with light emitting optical devices, such as laser diodes or optical fibers emitting light from a laser source. Prior to coupling, a light emitting facet of the optical device must be aligned with the small diameter core of the optical fiber. Alignment can be accomplished by positioning the light emitting facet of the optical device near the fiber core and moving the fiber relative to the optical device until laser light is observed to exit from the other end of the optical fiber. When the larger cladding layer is also waveguiding light, it is impractical to determine if the optical device is aligned with the core or the cladding layer.
A cumbersome method has been employed to assure that the cores, rather than the cladding layers, of the optical fibers are aligned with optical devices when the fibers have a low index of refraction protective coating. The protective layers are first stripped from an end segment of the fiber. The cladding layer will continue to waveguide light since the surrounding air has an index of refraction lower than the cladding layer. To prevent waveguiding in the cladding layer, the layer is surrounded by a high index of refraction liquid. Due to the thin cross-section of the fiber and the surface tension of the liquid, the liquid tends to bead rather than coat the fiber. Thus merely dipping the stripped section of the fiber in the liquid is insufficient, and the stripped fiber must be completely submerged in the liquid. Alignment is thus an awkward process. After alignment, the fiber section is removed from the liquid leaving a short unprotected segment of optical fiber susceptible to surface cracking.
High index of refraction coatings such as polyurethane have been applied to stripped optical fibers in attempts to eliminate waveguiding in the cladding layers. When polyurethane coatings have been applied in laboratory or field settings to align the fibers, the polyurethane is reacted on the stripped fibers. Incomplete covering, however, generally results which does not prevent waveguiding. The incomplete covering is due to beading of the polyurethane before it sets. In the few instances when a complete coating was obtained, the coating was thick and highly irregular and, therefore, not compatible with small diameter fiber systems and not suitable for coupling methods utilizing mechanical clamping.